In an optical pickup, optical information processing apparatus, optical attenuator, polarization conversion element, optical apparatus, projector optical system, a projector having any of the above, and various optical apparatus systems having any of the above, an optical element is used such as a polarization separation element capable of separating an optical path of an incident light based on a polarization direction of the incident light (see, for example, Patent Documents 1 through 7 and Non-Patent Documents 1 through 3).
As a known example of the optical element, FIG. 17 shows the polarization separation element 100 using a thin film 102 (see, for example, Patent Document 1) for separating the optical path. As shown in FIG. 17, the polarization separation element 100 includes two triangular prisms 101 and a thin film 102 formed on a boundary surface bonded between the two triangular prisms 101. By having this structure, the polarization separation element 100 separates an incident light “A” including a P-polarization component and an S-polarization component by transmitting the P-polarization component (A1) and reflecting the S-polarization component (A2). However, when the polarization separation element 100 as shown in FIG. 17 including the structure having the two triangular prisms 101 stuck together is used, the size of the polarization separation element 100 is large and the transmittance of the incident light “A” may largely fluctuate depending on the incident angle of the incident light “A”.
Polarization Separation Element Using Sub-Wavelength Convexo-Concave Structure (1)
On the other hand, as shown in FIGS. 18 and 19, there have been proposed optical elements 110 and 120, respectively, capable of serving as a double refraction wavelength plate, reflection prevention structure, polarization separation element and the like by having a sub-wavelength convexo-concave structure having a fine periodic structure having a pitch less than the wavelength of the incident light “A” (see, for example, Patent Document 1 and Non-Patent Documents 1 through 3). According to a report, the fluctuation of optical characteristics of the optical elements due to the change of the incident angle of the incident light is better-controlled, so that the optical characteristics of the optical elements are improved.
As shown in FIG. 18, the optical element 110 includes a material “nA” part and a material “nB” part, serving as a polarization separation element having the sub-wavelength convexo-concave structure “B” formed along a part of a grating period “Pt”. In the sub-wavelength convexo-concave structure “B”, an element boundary formed between the material “nA” part and the material “nB” part has a rectangular grating shape to form a fine periodic structure. The grating period of the fine periodic structure is defined as “pA” and a filling factor “fA” is defined as a ratio of the length of the material “nA” part to the grating period “pA”. Further, thicknesses of the material “nA” part and the material “nB” part are given as “dA” and “dB”, respectively.
On the other hand, as shown in FIG. 19, the optical element 120 includes a material “nA” part, a material “nB” part, and a material “nC” part, serving as a polarization separation element having the sub-wavelength convexo-concave structure “B” formed along a part of a grating period “Pt”. In this sub-wavelength convexo-concave structure “B”, there are triangular gratings formed as a part of a multilayer film including the material “nA” part and the material “nB” part. The material “nC” part is in contact with the multilayer film by an element boundary.
In the optical elements 110 and 120, the S-polarization component included in the incident light “A” travels in one direction as a zero-order diffracted light, and on the other hand, the P-polarization component included in the incident light “A” is separated into two directions as +/−1 diffracted lights. Namely, a light flux of a specific polarization direction (in this case, P-polarization component) is separated into two different directions. As a result, use efficiency of the light flux may be decreased.
Polarization Separation Element Using Sub-Wavelength Convexo-Concave Structure (2)
As an optical element capable of overcoming the problem, there has been proposed a polarization separation element 130 as shown in FIGS. 20A and 20B (see also, for example, Patent Document 1). FIG. 20A is a perspective view of the polarization separation element 130, and FIG. 20B is a cross-sectional view of the polarization separation element 130 cut along the line b-b in FIG. 20A.
As shown in FIG. 20A, in the polarization separation element 130, there is provided a diffraction grating “D” to perform diffraction separation. The diffraction grating “D”, as shown in FIG. 20B, has a one-dimensional blazed grating shape having the grating period “Pt” along the direction of the line b-b in FIG. 20A. Further, as shown in FIG. 20B, the grating shape of the cross-section of the diffraction grating “D” includes a first diffraction grating part “E” formed on a substrate “C” and having a blazed shape and a second diffraction grating part “F” formed on the first diffraction grating part “E”. Further, in the second diffraction grating part “F”, a sub-wavelength convexo-concave structure “B” is superimposed having a fine periodic structure across the entire light incident surface, the fine periodic structure having a pitch less than the wavelength of the incident light.
By having this structure, the light flux incident to the diffraction grating “D” is diffracted in different directions based on the polarization directions of the light flux; and further, the diffraction direction with respect to each polarization direction is directed mainly to a direction of a specific order only, so that the polarization separation element 130 may be used similar to a polarization separation element having thin films.
Patent Document 1: Japanese Patent Application Publication No. 2001-343512
Patent Document 2: Japanese Patent Application Publication No. 2008-257771
Patent Document 3: Japanese Patent Application Publication No. 2008-262620
Patent Document 4: Japanese Patent Application Publication No. 2008-276823
Patent Document 5: Japanese Patent Application Publication No. 2005-3758
Patent Document 6: Japanese Patent Application Publication No. 2004-37480
Patent Document 7: Japanese Patent Application Publication No. 2004-184505
Non-Patent Document 1: Hisao KIKUTA, Koichi IWATA “Formation of Wavefront and Polarization with Sub-Wavelength Gratings”, Optics, 1998, Vol. 27, No. 1, p. 12-17
Non-Patent Document 2: Yuzo ONO, “Polarizing Holographic Optical Element”, O plus E, 1991 March, No. 136, p 86-90
Non-Patent Document 3: Nao SATO “Forming Dielectric Photonic Crystals and Applied device”, O plus E, 1999 December, Vol. 21, No. 12, p 1554-1559